Sounds like the sensor for the burner is not functioning. The sensor sends a signal to the control pannel that says the burner is lit. If the control pannel doesn't get the signal, it shuts off the burner, then waits for the appropriate time to flush the unburned gas, then tries to reignite it. This cycle will repeat a specific number of times then, the fan may come on (depending on the mfg). Call a control HVAC diagnostician.